This invention relates to a process for forming a metal or metal compound coating on a face of freshly formed ribbon of hot glass during its advance through a coating station, which method comprises spraying such substrate at that station with coating precursor material from which said coating metal or metal compound is formed on the said substrate face. The invention includes apparatus which may be used for carrying out such a process.
During the last ten years such research has been carried out with the object of devising processes whereby coatings of high optical quality can be formed under normal factory conditions and at commercially acceptable cost. It has for long been recognized that a major problem is how to control the conditions at the coating station so that the coating has predetermined optical properties over the entire coated area. The optical properties obviously depend, inter alia, on the thickness of the coating and on its composition and structure and therefore these factors must be kept as uniform as possible over the area of the coating.
It is evident from available literature that considerable attention has been given to the direction in which the coating precursor material is sprayed, as one factor which can influence the results of the process. In some previously proposed processes the coating precursor material is sprayed so as to form a droplet stream with its axis perpendicular to the path of the substrate. In other known processes the droplets are discharged towards the substrate as a stream whose axis is inclined downwardly towards the substrate, either in the direction of the substrate advance or in the opposite direction. These various procedures are referred to e.g. in U.S. Pat. No. 4,125,391.
In certain spraying procedures previously proposed the precursor material is sprayed in a plurality of droplet streams distributed over the width of the area to be coated. In others precursor material is sprayed in a stream (hereafter called "scanning stream") which is repeatedly displaced across the substrate.
In the interest of promoting constant and predictable coating conditions it is established practice to control the spraying of the precursor material so that the dynamic conditions at the zone of impingement of the precursor droplets on the substrate are equilibrated as much as possible. But it has for many years been recognized that objectionable coating defects are liable to occur even when the momenta of the droplets just prior to contact with the substrate are very low. This is because the coating quality achieved is susceptible to the influence of conditions in the gaseous environment above the substrate. Therefore various ways have been suggested of acting upon the environment in order to remove substances which might impair the coating.
One example of a process which includes an environmental control measure is described in the aforementioned U.S. Pat. No. 4,125,391. In that process suction forces are created in exhaust ducting located to cause gases to flow away from the droplet stream and its impingement zone. The specification explains that this measure avoids or reduces the tendency for decomposition products to precipitate onto the substrate or the already formed coating from within the gaseous environment and that the said control measure makes it easier to form coatings of homogeneous structure and affording uniform coverage of the substrate, provided that, as a condition of the process, the suction forces are controlled so that they are substantially without effect on the paths of the precursor droplets towards the substrate.
Other processes including environmental control measures are described in U.S. Pat. Nos. 4,349,369 and 4,330,318. The process described in U.S. Pat. No. 4,349,369 is performed by using a perpendicularly directed or inclined scanning spray and propelling gas across the scanning path of the spray, out of line with the droplet stream(s). The purpose of this gas propulsion is to clean the atmosphere in the scanning path of the droplet stream or streams by removing vapour phase reaction products which might otherwise be entrained towards the substrate by the droplet stream(s). The specification indicates that the scavenging gas current can be discharged continuously provided its source is displaced in tandem with the source of the droplet stream(s). If on the other hand the scavenging gas is discharged from a stationary source then the gas is discharged intermittently, out of phase with the movement of the spray, so that the scavenging gas current does not encounter the droplets.
In the processes described in U.S. Pat. No. 4,330,318 the precursor droplets are sprayed in a direction inclined downwardly and forwardly or downwardly and rearwardly to the substrate ("forwardly" meaning in the direction of the substrate movement), and a jet of gas is discharged against the rear of the or each downwardly inclined droplet stream. This jetting of gas has the effect of reducing the incidence of coating defects giving rise to light diffusion in particular at the coating surface or at the interfacial zone between the coating and the substrate. The assumed reason for this effect is that the gas intercepts substances which would otherwise be entrained towards the substrate from the environment immediately to the rear of the droplet stream(s). The specification explains that the precursor droplets can be discharged in a plurality of streams from stationary sources distributed across the path of the substrate, in which case the gas can be jetted from one or more stationary orifices extending or distributed across the substrate path. Alternatively use can be made of one or more scanning droplet streams and the gas can be discharged from an orifice or orifices which is or are displaced transversely across the path of the substrate together with the droplet stream(s). The specification indicates that the strength of the gas jet(s) should not be such as to make the droplet stream(s) unsteady.
By taking one or more of the environmental control measures above referred to it is possible to form coatings of very good optical quanity under mass production conditions. The known processes are however of restricted scope in respect of the coating formation rates (expressed in terms of coating volume per unit time) which can be achieved. This is because the known processes are governed by the principle of keeping the dynamic conditions at the spraying zone as steady as possible. To meet this condition, the spraying is controlled to effect a quite gentle deposition of the droplets on the substrate and the environmental conditions are controlled so as to disturb the droplet trajectories as little as possible. The maximum permissible coating formation rate which can be achieved depends on various factors and can differ from one process to another but it is in all cases too low for achieving the coating formation rates which are sometime desirable. Obviously the coating formation rate determines the thickness of coating which can be formed on a glass ribbon travelling at a given speed through the coating station.